Cold cathode

ABSTRACT

A carbon film having an area of insulating material surrounded by an area of conducing material, and an area of material between the area of insulating material and the area of conducting material having a graded dielectric constant which varies from high to low from the area of insulating material to the area of conducting material.

CROSS REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 09/453,304 filed Dec. 2, 1999now U.S. Pat. No. 6,479,939, which is a CIP of Ser. No. 09/174,500 filedOct. 16, 1998 now U.S. Pat. No. 6,181,856.

TECHNICAL FIELD

The present invention relates in general to field emission devices, andin particular, to a cold cathode for use as a field emitter.

BACKGROUND INFORMATION

Cold cathodes are materials or structures that emit electrons with theapplication of electric fields without heating the emitter significantlyabove room temperature. Examples of cold cathodes are small metal tipswith sharp points that are fabricated together with a grid structurearound the tips such that an appropriate bias placed between the gridstructure and the tips will extract electrons from the tips whenoperated in a suitable vacuum environment (Spindt emitters).

Diamond, diamond-like carbon (DLC) and other forms of carbon films havealso been investigated for use as cold cathode electron emitters formany applications, such as flat panel displays, microwave deviceapplications, backlights for liquid crystal displays (LCDs), etc. Manydifferent techniques for growing the carbon films were tried resultingin a wide variety of carbon films. The mechanism for electron emissionfrom these carbon films is not clear and is the subject of muchinvestigation. What has been found consistently is that electrons arenot emitted uniformly from the carbon cold cathodes, but are insteademitted from specific areas or sites of the carbon film. These areas arethe emission sites (ES). The density of these sites in a unit area isreferred to as the emission site density (ESD).

Researchers recognized early on that the negative electron affinity ofthe hydrogen terminated <111> and <100> faces of diamond may beimportant. A material having negative electron affinity (NEA) means thatif an electron is in the conduction bands of the material, this electronhas no barrier to prevent it from leaving the material if the electrondiffuses to the surface having the NEA property.

The question for diamond has always been how to get an electron into theconduction band of diamond. This is not an easy question since diamondis an insulator with a very wide energy gap (5.5 eV) between theconduction band and the valence band. For an insulator at roomtemperature with this large a band gap, the population of electrons inthe conduction band is too small to support any substantial emissioncurrent. Researchers have speculated that the electrons are injectedinto the diamond from a back side contact.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates an apparatus for measuring the dielectric constant ofa material;

FIG. 2 illustrates results of one area of a carbon film showing thedielectric properties and the field emission properties;

FIG. 3 illustrates a field emitter device configured in accordance withthe present invention,

FIG. 4 illustrates a digital image of a single site emission currentimage;

FIG. 5 illustrates a digital image of the topography of a singleemission site taken simultaneously with the emission current image ofthe same site as illustrated in FIG. 4;

FIG. 6 illustrates a digital image of a non-contact topography image ofan emission site showing “grainy” distribution of physical parameters;

FIG. 7 illustrates a digital image of a contact topography of the singlesite illustrated in FIG. 6 showing a structure of “bumps”;

FIG. 8 illustrates a cross-section of emission current data from asingle emission site illustrating emission time dependence;

FIG. 9 illustrates a cross-section of fluctuations of a topographicimage of a single emission site whose current image is illustrated inFIG. 8;

FIGS. 10-12 illustrate graphs of wide variations in conductivity ofnon-emitting regions of a carbon film;

FIG. 13 illustrates a topographic image cross-section of an emittingcarbon film;

FIG. 14 illustrates a digital image of a portion of the emitting film asgraphed in FIG. 13;

FIG. 15 illustrates a graph of semiconductor-type behavior of anemission site of a film in accordance with the present invention; and

FIG. 16 illustrates a data processing system configured in accordancewith the present invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forthsuch as specific emitter types, etc. to provide a thorough understandingof the present invention However, it will be obvious to those skilled inthe art that the present invention may be practiced without suchspecific details. In other instances, well-known circuits have beenshown in block diagram form in order not to obscure the presentinvention in unnecessary detail.

Refer now to the drawings wherein depicted elements are not necessarilyshown to scale and wherein like or similar elements are designated bythe same reference numeral through the several views.

As disclosed in Ser. No. 09/174,500, the inventor produced films fortesting to characterize emission sites at better than 100 nm spatialresolution using a modified atomic force microscope (AFM).

Referring to FIG. 1, a modified scanning microscope was operated in twomodes. In the first mode the tip 104 was placed touching the surface ofthe sample 101 (mounted on sample holder 102) and scanned across thesurface 101 to measure the physical topography (AFM mode). The height ofthe tip 104 was detected by bouncing a light beam from light source 105off of the end of the tip 104 and reflected into a position detector106. The position of the light hitting the detector 106 is dependent onthe height of the needle tip 104. In another mode (scanning polarizationforce microscopy (SPFM) mode), the tip 104 was placed about 100 nm awayfrom the surface as shown in FIG. 1. A voltage bias from source 103 wasplaced on the needle tip 104 while the tip 104 was scanned across thesurface. By biasing the tip 104, an electric charge was placed on thetip 104 relative to the surface. The material reacted to the charge onthe tip 104 by placing charges on the surface in such a way as to formwhat appears to be an image charge inside the material 101. The strengthof the image charge is dependent on the dielectric constant (∈) of thematerial 101 as given by the equation

−q′=q*(∈−1)/(∈+1)

Here q′ is the magnitude of the image charge and q is the charge placedon the tip 104. Since the image charge is of opposite polarity to thecharge on the tip 104, an attractive force develops between the needletip 104 and the surface of the substrate 101. This force deflects theneedle 104. The magnitude of the force is detected by the position ofthe light hitting the detector 106. By scanning the needle tip 104across the surface, a mapping of the relative dielectric constant acrossthe surface is obtained. Simultaneously, if the bias on the tip 104 ishigh enough, electrons from the carbon film 101 can be field emittedfrom the surface of the sample 101 to the tip 104. By monitoring thecurrent to the tip 104, the emission sites of the carbon film 101 can belocated. Thus this instrument can map simultaneously the spatialemission properties of the sample 101 and the dielectric properties ofthe material 101, allowing a correlation of the results.

FIG. 2 shows the results of one area of the carbon film 101 showing boththe dielectric properties (left side image) and the field emissionproperties (right side image). What was discovered was that the fieldemission sites are correlated with specific dielectric properties of thesample 101. The features that are correlated to the emission sites arecharacterized by a dark area surrounded by a ring of bright area. Theylook like small volcanoes. Examples of these are features labeled A, B,and C. The emission sites are actually centered on the dark part of thevolcano features. This corresponds to an area of material having arelatively low dielectric constant surrounded by a ring of material 101having a relatively higher dielectric constant. Classically, thedielectric constant is related to the conductivity of the material. Wecorrelate the areas of relatively high dielectric constant to materialthat is more conductive. We correlate the areas of relatively lowdielectric strength to material that is more insulating. Since the filmwas grown by a diamond CVD process, we concluded that the insulatingmaterial was diamond and that the conductive ring was amorphous orgraphitic carbon.

We also noted that the dielectric distribution going towards the centerof these volcanoes was not abrupt but instead was gradual until itreached the dark center of the volcano. This suggests that thedielectric constant of the material 101 varies gradually towards thecenter of the volcano feature. In other words, the material surroundingthe diamond has a graded dielectric constant, the interfaces are notabrupt, but gradual. One of the emission sites (site A) has the volcanofeature as well as a well defined area of high dielectric constant nextto it.

We also noted that there are other features in the dielectric map thatdo not correspond to emission sites Two features marked E and F do nothave the dark centers of the volcano features. Another volcano-likefeature (labeled G) is not correlated with an emission site. Note thatthis feature also is not surrounded by a significant conducting ring asthe other features A, B, C.

Finally we noted that the intensity of emission from different sites wasnot uniform. The site marked D has the smallest emission intensity ofthe sites that emit. Its volcano features are hardly discernible.

Thus we discovered that a certain structure promotes electron fieldemission from the diamond films. These structures consist of a smalldiamond particle (less than 2000Å in diameter) surrounded by a materialthat has a dielectric constant that changes gradually in a volcano-likestructure. It is believed this structure is necessary to promoteinjection of electrons into the low dielectric material which ispresumably diamond. Once in the diamond conduction band, these electronshave little or no barrier for emission because of the low or negativeelectron affinity of the diamond surfaces.

The inventors have since performed additional tests on samples and havearrived at new discoveries. FIG. 4 illustrates an image of emissioncurrent of a single emission site measured using the SPFM mode with atip bias of +9.33 volts and a separation of 100 nm. FIG. 5 illustratesthe same single emission site illustrating a distinct SPFM topographyimage matching the same geometrical area of the emission current imagein FIG. 4. The correlation is similar to what was explained for FIG. 2,emission properties and dielectric properties are both imaged togetherover the same area of the sample.

FIG. 7 illustrates a digital image showing the topography of a singleemission site using the AFM mode, which illustrates a “grainy” structureof “bumps” of approximately 50-100 nm. FIG. 6 illustrates a topographyimage using the SPFM mode of the same emission site, which also shows a“grainy” distribution of physical parameters, which correlate to the“grainy bumps” from the AFM image in FIG. 7. The noise spikes within theFIG. 6 image are to be ignored.

When searching for these emission sites, an area of the sample measuringapproximately 6 micrometers×6 micrometers was searched. In general, ittook about three such general scans to locate an emission site. Aconclusion from the foregoing is that the emission site density isequivalent to one site per one hundred square micrometers or one millionemission sites per square centimeter.

Referring to FIG. 8, it has also been discovered by viewing thecross-section of the emission current data from a single emission sitethat the emission is time-dependent with a lateral resolution of 50 nm.The cross-section of the image data in FIG. 8 was taken using the SPFMmode. There are two items to note in this scan: (a) the image does notrepeat itself on consecutive scans, and (b) the feature sizes along thelength scale are sharper than what is expected given the resolution ofthe measurement in SPFM mode. FIG. 9 illustrates the simultaneoustopographic image in SPFM mode of such a single site The features inthis scan also do not repeat in consecutive scans and are also sharperthan expected given the resolution of the instrument in this mode. Thearea of sharp features in FIG. 8 is correlated with the area of sharpfeatures in FIG. 9. These two figures show that the emission current ischanging with time and the surface potential due to surface charging isalso changing with time.

FIGS. 10-12 show that non-emitting regions of a sample do not showsemiconductor interface behavior, but instead a wide variation inconductivity from a perfect insulator to nearly ohmic behavior. FIG. 15,however, illustrates a contact I-V spectra of an emission site, whichshows a semiconductor type behavior with a relatively large band gap.

FIGS. 13 and 14 illustrate that there is typically no geometricenhancement at the vacuum/film interface. The image in FIG. 14 and thegraph in FIG. 13 were taken using a contact AFM mode with a +6 voltbiased tip.

Some conclusions can now be made. The emission sites are formed ofgeometric grainy bumps. However, such emission sites do not havemicrotips, but are very relatively flat. The sharpest features have arise of ˜±20 nm over a distance of ˜1.0 nm (˜1000 nm). This correspondsto an enhancement of 2°% or less, which is very flat compared tomicrotip cathodes. A location that emits exhibits a semiconductorbehavior with a wide band gap. Furthermore, the gradient portiondescribed previously with respect to FIG. 2 is time-dependent so thatnonactive sites, such as sites E and F become active at a later time,and the active sites, A, B, and C, will become inactive. Furthermore,such periods of activity and inactivity may be coupled and mayoscillate, as the transitional intermittent interfaces between thegrainy bumps behave as semiconductors whereby a charge builds up and isthen emitted as electrons, resulting in an emission site. Subsequentlythe interface between the bumps loses the charge and must again chargeup to a threshold limit. During the charge up period, the emission siteis inactive. For further discussion, refer to J. Robertson, “Mechanismof Electron Field Emission From Diamond and Diamond-Like Carbon,” IVMC98, pp. 162-163, which is hereby incorporated by reference herein.

The areas of grainy bumps may be areas of carbon growth on the surfaceof the substrate. It is known by scanning electron microscope imagesthat the carbon film is not continuous across the surface of the sample.The grains within the bump may be grains of diamond plus grains ofgraphite in an amorphous carbon matrix.

Please note that the carbon emitter of the present invention maycomprise any known carbon-based field emission device, including carbonfilms, microtip structures, and carbon nanotubes.

Referring next to FIG. 3, there is illustrated field emitter device 80configured with a film produced in accordance with the inventiondiscovered above. Device 80 could be utilized as a pixel within adisplay device, such as within display 938 described below with respectto FIG. 16.

Device 80 also includes anode 84, which may comprise any well-knownstructure. Illustrated is anode 84 having a substrate 805, with aconductive strip 806 deposited thereon. Then, phosphor layer 807 isplaced upon conductive film 806. An electrical potential V+ is appliedbetween anode 84 and cathode 82 as shown to produce an electric field,which will cause electrons to emit from film 501 towards phosphor layer807, which will result in the production of photons through glasssubstrate 805. Note that an alternative embodiment might include aconductive layer deposited between film 501 and substrate 101. A furtheralternative embodiment may include one or more gate electrodes (notshown).

As noted above, field emitter device 80 may be utilized within fieldemission display 938 illustrated in FIG. 16. A representative hardwareenvironment for practicing the present invention is depicted in FIG. 16,which illustrates a typical hardware configuration of workstation 913 inaccordance with the subject invention having central processing unit(CPU) 910, such as a conventional microprocessor, and a number of otherunits interconnected via system bus 912. Workstation 913 includes randomaccess memory (RAM) 914, read only memory (ROM) 916, and input/output(I/O) adapter 918 for connecting peripheral devices such as disk units920 and tape drives 940 to bus 912, user interface adapter 922 forconnecting keyboard 924, mouse 926, speaker 928, microphone 932, and/orother user interface devices such as a touch screen device (not shown)to bus 912, communication adapter 934 for connecting workstation 913 toa data processing network, and display adapter 936 for connecting bus912 to display device 938. CPU 910 may include other circuitry not shownherein, which will include circuitry commonly found within amicroprocessor, e.g., execution unit, bus interface unit, arithmeticlogic unit, etc. CPU 910 may also reside on a single integrated circuit.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims.

What is claimed is:
 1. A method of operating a field emission device,comprising the steps of: providing an emitter deposited on a substrate;and applying an electric field to the emitter to thereby cause anemission of electrons from an emission site comprising grains ofconducting and non-conducting material, wherein the emission from theemission site is time dependent.
 2. The method as recited in claim 1,wherein the emitter is a carbon emitter.
 3. The method as recited inclaim 1, wherein material in the emission site exhibits a wide band gap.4. A method of operating a field emission device, comprising the stepsof: providing an emitter deposited on a substrate; and applying anelectric field to the emitter to thereby cause an emission of electronsfrom an emission site comprising grains of conducting and non-conductingmaterial, wherein the emission from the emission site is intermittent.5. A method of operating a field emission device, comprising the stepsof: providing an emitter deposited on a substrate; and applying anelectric field to the emitter to thereby cause an emission of electronsfrom an emission site comprising grains of conducting and non-conductingmaterial, wherein the emission site is surrounded by non-emittingregions having a wide variation in conductivity from insulative tonearly ohmic.
 6. A method of operating a field emission device,comprising the steps of: providing an emitter deposited on a substrate;and applying an electric field to the emitter to thereby cause anemission of electrons from an emission site comprising grains ofconducting and non-conducting material, wherein the emission siteappears as a conglomerate of grains as shown on an AFM image on a scaleof 1 micrometer×1 micrometer or larger.
 7. A method of operating a fieldemission device comprising the steps of providing a carbon emitter on asubstrate, wherein the carbon emitter includes a first emission sitehaving a plurality of bumps with interfaces therebetween, and a secondemission site having a plurality of bumps with interfaces therebetween;and applying an electric field to the carbon emitter to thereby cause anemission of electrons from the first emission site, wherein theapplication of the electric field to the carbon emitter results inemission of electrons from the second emission site a time period afterelectron emission from the first emission site, wherein the firstemission site is not emitting electrons while the second emission siteis emitting electrons.